In recent years, research and development of a high density optical disc system capable of recording and/or reproducing information by using a violet semiconductor laser with a wavelength of about 400 nm, are advancing rapidly. As an example, in the case of an optical disc on which information is recorded and/or reproduced under the specifications that NA is 0.85 and a light source wavelength is 405 nm, namely, in the case of the so-called Blu-ray Disc (hereinafter, BD), it is possible to record information of about 23 through 27 GB per layer for an optical disc with a diameter of 12 cm, which is same in size as DVD (NA is 0.6, wavelength of a light source is 650 nm, and memory capacity is 4.7 GB). As another example, in the case of an optical disc on which information is recorded and/or reproduced under the specifications that NA is 0.65 and a light source wavelength is 405 nm, namely, in the case of the so-called HD DVD (hereinafter, HD), it is possible to record information of about 15 through 20 GB per layer for an optical disc with a diameter of 12 cm. With respect to BD, coma caused by inclination (skew) of an optical disc is increased. Therefore, an amount of the coma caused by skew is lowered by designing the protective layer to be thinner than that in the case of DVD (where the thickness is 0.1 mm, compared with 0.6 mm for DVD). Hereafter, the optical disc of this kind is called “a high density optical disc” in the present description.
In the meantime, a value as a product for an optical disc player/recorder is not sufficient when the optical disc player/recorder only can record and/or reproduce information properly for the high density disc of this kind. In view of the realities that DVDs on which various types of information are recorded are on the market at present, only conducting information recording and/or information reproducing is not sufficient for the high density optical disc, and ability to conduct information recording and/or information reproducing properly also for DVD owned by a user, for example, enhances commercial value as the optical disc player/recorder for the high density optical disc. With the aforesaid background, an optical pickup device to be built in the optical disc player/recorder for a high density optical disc is requested to have capability to conduct information recording and/or information reproducing properly for both a high density optical disc and DVD also, while maintaining a compatibility.
As a method to be capable of recording and/or reproducing information for both of the high density optical disc and DVD also while maintaining compatibility, there is considered a method to switch an optical system for a high density optical disc and an optical system for DVD selectively, depending on recording density of an optical disc for information recording and/or information reproducing. However, this method is disadvantageous for downsizing because a plurality of optical systems are needed, and this method cannot avoid cost increase.
Therefore, it is preferable to simplify construction of the optical pickup device, and to reduce the number of optical parts constituting the optical pickup device by providing an optical system for a high density optical disc and that for DVD as a common optical system even in the optical pickup device with a compatibility, for achieving low cost. Most advantageous ways for simplification and low cost of the construction of the optical pickup device includes providing an objective lens arranged to face an optical disc as a common optical element and further making this objective lens a single lens. As an objective lens that is common to optical discs in plurality of types each being different in recording/reproducing wavelength, there is known an objective lens that has, on its surface, a diffractive structure having wavelength-dependency of an spherical aberration, and that corrects a spherical aberration caused by a difference in recording/reproducing wavelength and in a protective layer thickness, by utilizing the wavelength-dependency of this diffractive structure.
Patent Literature 1 discloses an objective lens in a single lens structure that can record and/or reproduce information compatibly for a high density optical disc, DVD and CD.    Patent Literature 1: JP-A No. 2006-185576